Preparation of thorium silicate hydrosols



i high burn-up'of fuel is possible.

3,l9,583 Patented Dec. 1, 196% This invention relates to heavy metalsilicate sols with appreciable stability under extreme hydrothermalconditions. In one particular embodiment it relates to thorium silicatesols suitable for use in the fuel systems of aqueous homogeneousreactors.

Aqueous homogeneous reactors may be one of three types: Burner reactors,converter reactors or breeder reactors. I Burner reactors are those inwhich fissionable materials-are consumed as fuels but virtually no fuelis generated. Converter reactors are those which produce a differentfissionable fuel from that which is destroyed in the fission process.Breeder reactors are those which produce more of the'same type, offissionable fuel as is being consumed in the reactor. A converterreactor be comes a breeder reactor if there is a net gain in theproductionof fissionable fuel and this fuel is subsequently burned inthe reactor. l

The nuclear reactions involved in the breeder reactor using a mixedthoria-uram'a fuel are typical and are well known. In a two regionreactor, for example, a core of uranium solution is surrounded by ablanket of thorium 232. As the uraniumin the core fissions, it gives offneutrons, some of which are' absorbed by the thorium 232 to convert itto thorium 233. Thorium 233 decays with a half life off 33.3 minutes toyield protactinium 233 which in turn decays to uranium 233. The uranium233 is a fissionable uranium isotope and is itself a suitable fuel.These breeder reactors may also be designed as single region reactorswhich contain a homogeneous mixture of fissionableand fertile materialin a moderator. These reactors differ from'the single region reactor inthat they have larger reactor diameters in order to minimize neutronlosses. They normallycontain the fuel plus fertile material inconcentrations as high as 300, g'ramsper liter. Aqueous homogeneousreactors haveseveral advantages over the conventional type .of reactorsused in nuclear power'development. I These advantages stern partly from.

the fluid nature of the fuels and partly from the homo fuel fluid, thereis essentially no heat transfer barrier between the fuel and thecoolant; These reactorsIalso-compare favorably with heterogeneousreactors in that the Because the fuel is liquid, continuous removal ofpoisons that cause radiation damage to fuel elements is possible and newfuel can be continually added to the system thereby permitting unlimitedburn-up. -Neutron economyin the liquid fuel system is improved .byeliminating. the absorption of neutrons by thecl'adding' and thestructural materials which are present intbe reactor core of theheterogeneous reactors.

1 rapid removal of fission product poisons.

In so'ineof the reactor systems of the prior' artpuranyl The design ofthesereactors makespossible United States Patent Office ration.

homogeneous reactors. These solutions have not been particularlysatisfactory as neutron sources because they are corrosive attemperatures of 250 to 300 C. and. at these temperatures have been foundto be unstable.

.--'-'Slurries have certain obvious disadvantages as fuels in that theyrequire constant agitation to prevent solids sepa The equipment alsotends to be eroded by the movement of these solid particles in the fuelelement of the reactor. Attrition is also frequent y a problem in thatthese solidfuels tend to be subject to attrition in the op eration ofthe reactor.

It has been recognized that these problems can ,be

solved by using urania sols or thoria-urania sols as the fuel in aqueoushomogeneous reactors of the type set out above. Sols have the advantageof being homogeneous fluids and have been found to avoid thedisadvantages that are present where urania or thoriarurania slurriesare used. There is, for example, no need to furnish agitation to preventsolids separation. Because of their small size, these particles are notsubject to attrition and the problem of erosion of equipment becomesuniniportant. Sols also have relatively low viscosities and thus can beeasily pumped.

In accordance with the presentinvention, we have discovered that thoriumsilicate particles of desirable micelle size can be prepared as thoriumsilicate sols (and if expedient to do so, clad with a layer of silica)and the entire sol stabilized bythe addition of an alkali metalhydroxide. The silica and alkali metal have desirably lowneutron-capture cross-section and do not interfere with reactionsinvolving the thorium. However, in order to obtain the desiredcharacteristics, it is necessary that the various components be'preparedin a carefully controlled manner and brought together in a specifiedorder under likewise carefully controlled conditions.

In order to obtain a final thorium silicate sol of the necessaryhydrothermal stability, low viscosity and other desirableproperties, itis desirable that the thorium silicate particles be spheroidal orsubstantially so. In addition, the thorium silicate particles ofthepresent invention should preferably be of generally uniform szehaving a particle size generally less than about 390 rnillimicrons.Suitable silicate sols can be prepared by hydrolyzing concurrently adilute thorium salt and an organicv silicate while maintaining thesystem at an elevated temperature.

This procedure is essentially a co-hydrolysis in which both hydrolysisreactions are proceeding concurrently and are interacting. Thus, thoriumin the plus IV oxidation state is being hydrolyzed under conditions thatnormally give thoria, and an organic silicate isbeing hydrolyzed underconditions that normally give silica. When both of thesehydrolysisreactions are proceeding concurrently, the path of the reaction ischanged and insteadof thoria and .silica being obtained as the products,a very stable compound, thorium orthosilicatetThsifi results. Theinteraction "of the various intermediate products of both hydrolysesmust occur under the proper conditions and I rates. The thoriumorthosilicate productis. colloidal/- The hydrolysis of the thorium intheplus W state may be accomplished by: (1.) anion removal byelectrodialysis; (2) urea hydrolysis, 017(3) by using a dilute solutionof thorium in the'plus lV-0Xidation state in the presence ofanorganicsilicate. g In the. present process, we start with an aqueoussolu tion ,of athorium salt of a strongmonobasic acid, pref-- erablywith a salt that is sufficiently soluble that the dialysis or directreaction with an organic orthosilicate will proceed satisfactorily.Thorium nitrate, thorium chloride and thorium perchlorate are suitablefor this purpose. The chlorides, however, exhibit a tendency towardbeing corrosive at elevated temperatures and, therefore, the chloridecontent should be reduced as much as possible. For this reason, if thethorium silicate sol, for example, is being prepared to be used for areactor blanket, it is normally preferred to use thorium nitrate in thereaction with the organic silicate. Trace nitrate impurities which mightremain in the sol are not as harmful as comparable amounts of chlorideswould be. Thorium silicate sols prepared by any of the foregoingtechniques are characterized by relatively dense, generally sphericalparticles having colloidal dimensions and exhibiting little tendency toagglomerate at ambient temperatures. The hydrothermal stability atelevated temperatures of these thorium silicate sols may be improved bycoating the thorium silicate sols with silica. The hydrolysis isnormally carried out at reflux temperature but the reaction wouldproceed satisfactorily at temperatures from about 70 to about 150 C.

The generally spherical, colloidal, densified thorium silicate particlesprepared as described above can be clad with a protective layer ofsilica, whenever it is expedient to do so, by simply adding a slightexcess of the organic silicate after the silicate is formed, heatinguntil hydrolyzed, and stabilizing by maintaining a prescribed pH byadding alkali hydroxides as needed.

Since neutron-capture is a nuclear process dependent on atomicconcentration and since silicon is a relatively light element having anatomic weight of only 28 as compared to an atomic weight of 232 forthorium, the thoria to silica ratio should be as high as possible.However, even in ratios as low as 1:1, the presence of silicon does notreduce the emciency excessively because its thermal neutron-capturecross-section, that is, its tendency toward neutron-capture, is, forexample, only 0.13 barn as compared with 7.0 barns for thorium. Thus, ina thoria to silica Weight ratio of 1:1 (atomic ratio of 0.227z1) thesilicon will capture only about 8% of the neutrons and the thorium willcapture 92%; at a 3 :1 ratio the silica will capture only about 2.7%

The specific conductance is generally used as a measure of the anionicimpurities and should be kept below 10* mho/ cm. in the final product.

Where the anion content is high enough to get an undesirable product,further purirfication is normally carried out. This purification canbest be effected by heating the alkali sol under non-evaporativeconditions, that is, under total reflux or in an autoclave to insurerelease of the anions from the micelles and then contacting with adeionizer to remove the electrolytes. If the alkali metal ions areremoved, the alkalinity must be restored by adding more alkali metalhydroxide. The resulting pH should be from about 7.0 to about 9.5. Thus,except for the stabilizing alkali metal cations, the resulting solutionis substantially electrolyte free.

Since sols of this type tend to coagulate, often irreversibly, in thepresence of electrolytes, care must be taken to keep the electrolytecontent at a minimum. A convenient method of measuring concentration ofthe undesired ionic material is specific conductance. For the sols ofthe present invention, specific conductance will usually range between10- and 10- mho/cm. The stability of any given sol is improved byreduction in the ionic content; therefore, specific conductances in thelower part of the range are preferred.

The sol prepared as described can be concentrated by evaporation to asolids content of up to 45% total solids in the usual case. It ispreferred to add fresh sol continuously during evaporation to avoiddeposition of the solid material on the sides of the'vessel. Thefinished sol may be diluted to any lower solids content by the additionof deionized water or water of low ionic content.

Specific conductance is measured at 25 C. and one kilocycle using astandard conductivity bridge with a cell inserted in one arm. The cellconstant is determined using KCl solutions of 0.01 normality (theconductance of which is ascertained from the conductivity tables) andusing the equation where K =cell constant in cm.-

R=bridge resistance in ohms L=specific conductance in mhos/ cm. of thestandard KCl solution The conductance L of the sol in question can bedetermined by measuring its resistance in the same cell and using theequation where K :cell constant in CH1. 1 R=resistance in ohms Thethorium content of our sols was determined by fluorescent X-rayspectroscopy and by standard gravimetric techniques. Electronmicrographs were made by the standard techniques.

In the present disclosure, we have referred to the use of alkali metalhydroxides and, specifically, to sodium hydroxide although otherpreparations may be used. The only limitations in the selection of thebase resides in the fact that the base should be composed of low thermalneutron cross-section elements and be stable under reactor conditions.

in referring to our dispersions of thorium silicate in water, we intendto include heavy water as well as natural Water.

In referring to organic orthosilicates, we mean to include organiccompounds which will produce silica on hydrolysis. Suitable compoundsinclude various alkoxy silanes, preferably with alkoxy groups smallerthan butoxy, in order that the compound be not too resistant tohydrolysis. The preferred compound is ethyl orthosilicate due both tosuitable hydrolysis rates and availability. 1

The hydrolysis will proceed satisfactorily if the number of moles oforganic silicate present in the solution is about equal to or slightlyin excess of the number of moles of the thorium salt present.

While our invention is primarily concerned with the preparation ofthorium silicate sols, the process disclosed should be consideredapplicable to other metal salts hydrolyzing at low pH such as aluminum,iron, titanium, zirconium, uranium, plutonium, etc.

The present invention will be further explained by the followingillustrative but non-limiting examples.

EXAMPLE I A charge of 1760 grams of a thorium nitrate solutioncontaining 5% "H10 in deionized water was placed in a vessel,hereinafter designated as a densification vessel, and heated to 97.8" C.The solution was circulated at a rate of approximately 60 cc. per minutethrough a water cooler to reduce the temperature to 3040 C. and thenthrough the cathode compartment of a cell divided by an ion exchangemembrane of Amberplex A1. The electrode compartments each'had a capacityof 350 cc. and each was equipped with a stirrer. Platinum electrodeswere 'positioned'on each side of the membrane at a distance of about Msinch from the membrane.

Small quantities of ethyl orthosilicate were added to the densificationvessel over the period of the 11m. A total of ml. of ethyl orthosilicatewas added in 5 and 10 ml. increments over an eight hour period. The

temperature of the densification vessel was maintained through a heatexchanger whereit was heated to 97 to 198 C. and then returned to the'densification vessel.

Evaporation losses were minimized by equipping. the cell with acondenser and by periodically adding deionized waterto take care ofunavoidable losses.

Circulation of the solution was continued over a total period of 10.5hours with overnight interruptions at eighthour intervals during whichperiod the solution was cooled to room temperature. During electrolysis,the amperage dropped from 10 to a value or" about .4 and the pH rosefrom 2.7 to about 4.6. The sol (1752 g.) had a density of 1.04 grams/co,a pH of 3.6, a specific conductance of 9.6 10 rnhos/cm. and contained4.02% Th and 1.11% SiO In thiscondition, the sol was stable at 150 C.The electron diffraction pattern of this s01 was identified as that ofthorite, a thorium oithosilicate.

Electron micrographsrevealed well distributed particles having a weightmedium in the range of 100 me.

To 500 grams of the above sol, 12.7 grams of additional ethylorthosilicate, suflicien-t to bring the siO /ThO ratio of the system to2.2, was added. The addition was followed by 36 hours of refluxing afterwhich the pH was adjusted to a pH of 9 with a 1 normal solution ofsodium hydroxide. After refluxing for an additional 24 hours, the solwas deionized by passing it through a mixed bed resin and the pH wasadjusted to 8.2 bythe addition of a 1 normal solution of sodiumhydroxide. The sol was autoclaved for 24 hours at 150 C. The thoriaconcentration was determined at this point (density 1.20) and found tobe 11.1%. The sol was deionized for the second time to reduce thespecific conductance. After the pH was adjusted to-8, the specificconductance was found to be 2.5 X l0 mhos/crn. The sol was autoclaved at275 C. for various periods of time in order to determine itshydrothermal stability.

A sample'of the above sol was treated by centrifuging and redispersinginto deionized water to yield a sol having a density of 1.32, a pH of3.1, and a specificconductance of 1.28 miles. In this condition, the solwas stable for at least 140 hours at a temperature of 275 C. The resultsof these tests together with some of" the physical characteristics ofthe sol are given in Table I below. V

7 Table I Density at 25 C. (grams/ml.) 1.18 PH -n v 8.0 Specificconductance (mhos/cm.) 2.5X10

' thorium silicate sol suitable for use in the fuel system of an aqueoushomogeneous reactor. 1

A charge of 21.3 grams (.038 mole) of thorium nitrate Th(NO ).-4H O, 4.6grams (.076 mole) of urea and 7.9 grams (.038 mole) of ethylorthosilicate was added to a two liter flask with 366 grams of water.These com- 6 EXAMPLE III A thorium silicate sol was prepared using athird process.

A charge of 21.3 grams of thorium nitrate and 7.9 grams of ethylorthosilicate were added to a two'liter flask together with 971 grams ofwater. The mixture'was refluxed at 100C. for 16 hours. At the end ofthis period, the slurry was removed and a sample submitted for electronmicroscope study. The sol gave an electron dillraction pattern forthorite. The results of these tests together with some of the physicalcharacteristics of this and other sols prepared by the same techniqueare given in Table III.

, Table'lll Percent ThO (by method of preparation) 1 Density at C.(grams/ml.) 1.00

pH -L. 9.7

Obviously many modifications and variations of the invention ashereinabove set forth may be made without departing from the essence andscope thereof and only such limitations should be applied as areindicated in the appended claims.

We claim: H

1. The method of preparing a thorium silicate sol by 7 concurrenthydrolysis of a solution of a thorium salt and ponents were refl xedforv 16 hours at 100 C. The prodsol. .The solwas autoclaved at 275 C. todetermine its hydrothermal stability. The results of thesetests-togcther with some of the physical characteristics of the Table 11sols preparedin this manner are given in Table II below.

an organic silicate comprising the steps of preparing a solution of atetravalent thorium salt selected from the group consisting of thoriumnitrate, thorium chloride and thorium perchlorate in water, addingalkoxy silanes selected from those having alkoxy'groups smaller thanbutoXy in an amountabout equal to the number of moles ofthorium saltpresent to the solution, withdrawing a portion of said solution,dialyzing said portion to remove anions, returning said portion to alarger body of solution maintained at about -to-150 C. and continuingsaid withdrawal, dialysis and addition back until said body of solutionis essentially electrolyte free and recovering the product thorite sol.

2. The method of preparing a thorium silicate sol by concurrenthydrolysis of a solution of a thorium salt and an orgmic siliratecomprising the steps of preparing a solution of thorium nitrate inwater, adding ethyl orthosilicate to said solution in an amount aboutequal to the number of moles of thorium nitrate present, withdrawing aportion of said solution, dialyzing said solution to remove anions andreturning said portion .to a larger body of solution maintained at about(3., con

tinuing said withdrawal, dialysis and addition back until said body ofsolution is essentially electrolyte free, adding an additionalquantity'of ethyl orthosilicate'to the solution and recovering theproduct silica-coated thorite sol.

3. The method of preparing a menu: sol by concurrent hydrolysis of asolution of a thorium salt and an. organic silicate which comprises thesteps of preparing an aqueous solution of thorium nitrate, adding aquantity of urea about equal to the molar quantity of -thorium'nitratepresent anda quantity of ethyl'orthosilicate to the solution about equalto the number of molesbf thorium nitrate present, heating the resultantsolution atabout 70 to C. for a period of time suflicient to completethe hydrolysis and recovering the product thorite sol. 4. The method ofpreparing-a thorite sol by concurrent hydrolysis of a solution of athorium salt and an organic silicate which comrpises preparing asolution of thorium nitrate, adding a quantityof urea, equal, to abouttwice the molar quantity of thorium nitrate present and a quantity ofethyl orthosilicate equal to the number of 7 moles ofthorium nitratepresent, refluxing the resulting I mixture-at a" temperature ofabout 100C. roraperiod; a f of about 20 hours and recovering the product thoritesol. a 75 flteferences on foliowingpage) References Cited by theExaminer UNITED STATES PATENTS OTHER REFERENCES Mellor: ComprehensiveTreatise on Inorganic and Stark Theoretical Chemistry, vol. 6, p. 859(1925).

Prescott 117 46 Weiser: C0l1oidal Salts, 1st edition, pp. 323, 324

Schlesmger et a1. 23--14.5

Her 252 313 AEC Document K295, Part II, page 115 (abstract Her 252 313928), Mar. 1, 1955.

figiggg gf 2 CARL D. QUARFORTH, Primary Examiner.

Barton 252 3o1.1 REUBEN EPSTEIN, JULIUS GREENWALD,

Smith-Johannsen 252-313 Examiners.

1. THE METHOD OF PREPARING A THORIUM SILICATE SOL BY CONCURRENTHYDROLYSIS OF A SOLUTION OF A THORIUM SALT AND AN ORGANIC SILICATECOMPRISING THE STEPS OF PREPARING A SOLUTION OF A TETRAVALENT THORIUMSALT SELECTED FROM THE GROUP CONSISTING OF THORIUM NITRATE, THORIUMCHLORIDE AND THORIUM PERCHLORATE IN WATER, ADDING ALKOXY SILANESSELECTED FROM THOSE HAVING ALKOXY GROUPS SMALLER THAN BUTOXY IN ANAMOUNT ABOUT EQUAL TO THE NUMBER OF MOLES OF THORIUM SALT PRESENT TO THESOLUTION, WITHDRAWING A PORTION OF SAID SOLUTION, DIALYZING SAID PORTIONOF REMOVE ANIONS, RETURNING SAID PORTION TO A LARGER BODY OF SOLUTIONMAINTAINED AT ABOUT 70 TO 150*C. AND CONTINUING SAID WITHDRAWAL,DIALYSIS AND ADDITION BACK UNTIL SAID BODY OF SOLUTION IS ESSENTIALLYELECTROLYTE FREE AND RECOVERING THE PRODUCT THORITE SOL.